2.4.2. Using do-notation at the prompt

GHCi actually accepts statements
rather than just expressions at the prompt. This means you can
bind values and functions to names, and use them in future
expressions or statements.

The syntax of a statement accepted at the GHCi prompt is
exactly the same as the syntax of a statement in a Haskell
do expression. However, there's no monad
overloading here: statements typed at the prompt must be in the
IO monad.

Prelude> x <- return 42
Prelude> print x
42
Prelude>

The statement x <- return 42 means
“execute return 42 in the
IO monad, and bind the result to
x”. We can then use
x in future statements, for example to print
it as we did above.

If -fprint-bind-result is set then
GHCi will print the result of a statement if and only if:

The statement is not a binding, or it is a monadic binding
(p <- e) that binds exactly one
variable.

The variable's type is not polymorphic, is not
(), and is an instance of
Show

.

Of course, you can also bind normal non-IO expressions
using the let-statement:

Prelude> let x = 42
Prelude> x
42
Prelude>

Another important difference between the two types of binding
is that the monadic bind (p <- e) is
strict (it evaluates e),
whereas with the let form, the expression
isn't evaluated immediately:

Hint: you can also use let-statements
to define functions at the prompt:

Prelude> let add a b = a + b
Prelude> add 1 2
3
Prelude>

However, this quickly gets tedious when defining functions
with multiple clauses, or groups of mutually recursive functions,
because the complete definition has to be given on a single line,
using explicit braces and semicolons instead of layout:

Such multiline commands can be used with any GHCi command,
and the lines between :{ and
:} are simply merged into a single line for
interpretation. That implies that each such group must form a single
valid command when merged, and that no layout rule is used.
The main purpose of multiline commands is not to replace module
loading but to make definitions in .ghci-files (see Section 2.9, “The .ghci file”) more readable and maintainable.

Any exceptions raised during the evaluation or execution
of the statement are caught and printed by the GHCi command line
interface (for more information on exceptions, see the module
Control.Exception in the libraries
documentation).

Every new binding shadows any existing bindings of the
same name, including entities that are in scope in the current
module context.

WARNING: temporary bindings introduced at the prompt only
last until the next :load or
:reload command, at which time they will be
simply lost. However, they do survive a change of context with
:module: the temporary bindings just move to
the new location.

HINT: To get a list of the bindings currently in scope, use the
:show bindings command:

Prelude> :show bindings
x :: Int
Prelude>

HINT: if you turn on the +t option,
GHCi will show the type of each variable bound by a statement.
For example:

2.4.3. Multiline input

Apart from the :{ ... :} syntax for
multi-line input mentioned above, GHCi also has a multiline
mode, enabled by :set +m,
in which GHCi detects automatically when the current statement
is unfinished and allows further lines to be added. A
multi-line input is terminated with an empty line. For example:

Prelude> :set +m
Prelude> let x = 42
Prelude|

Further bindings can be added to
this let statement, so GHCi indicates that
the next line continues the previous one by changing the
prompt. Note that layout is in effect, so to add more bindings
to this let we have to line them up:

Prelude> :set +m
Prelude> let x = 42
Prelude| y = 3
Prelude|
Prelude>

Explicit braces and semicolons can be used instead of
layout, as usual:

As with ordinary variable bindings, later definitions shadow
earlier ones, so you can re-enter a declaration to fix a problem
with it or extend it. But there's a gotcha: when a new type
declaration shadows an older one, there might be other
declarations that refer to the old type. The thing to remember is
that the old type still exists, and these other declarations still
refer to the old type. However, while the old and the new type
have the same name, GHCi will treat them as distinct. For
example:

The old, shadowed, version of T is
displayed as main::Interactive.T by GHCi in
an attempt to distinguish it from the new T,
which is displayed as simply T.

2.4.5. What's really in scope at the prompt?

When you type an expression at the prompt, what
identifiers and types are in scope? GHCi provides a flexible
way to control exactly how the context for an expression is
constructed. Let's start with the simple cases; when you start
GHCi the prompt looks like this:

Prelude>

Which indicates that everything from the module
Prelude is currently in scope; the visible
identifiers are exactly those that would be visible in a Haskell
source file with no import
declarations.

If we now load a file into GHCi, the prompt will change:

Prelude> :load Main.hs
Compiling Main ( Main.hs, interpreted )
*Main>

The new prompt is *Main, which
indicates that we are typing expressions in the context of the
top-level of the Main module. Everything
that is in scope at the top-level in the module
Main we just loaded is also in scope at the
prompt (probably including Prelude, as long
as Main doesn't explicitly hide it).

The syntax
*module indicates
that it is the full top-level scope of
module that is contributing to the
scope for expressions typed at the prompt. Without the
*, just the exports of the module are
visible.

We're not limited to a single module: GHCi can combine
scopes from multiple modules, in any mixture of
* and non-* forms. GHCi
combines the scopes from all of these modules to form the scope
that is in effect at the prompt.

NOTE: for technical reasons, GHCi can only support the
*-form for modules that are interpreted.
Compiled modules and package modules can only contribute their
exports to the current scope. To ensure that GHCi loads the
interpreted version of a module, add the *
when loading the module, e.g. :load *M.

The full Haskell import syntax is supported, including
hiding and as clauses.
The prompt shows the modules that are currently imported, but it
omits details about hiding,
as, and so on. To see the full story, use
:show imports:

Note that the Prelude import is marked
as implicit. It can be overriden with an explicit
Prelude import, just like in a Haskell
module.

Another way to manipulate the scope is to use the
:module command, which provides a way to do
two things that cannot be done with ordinary
import declarations:

:module supports the
* modifier on modules, which opens the
full top-level scope of a module, rather than just its
exports.

Imports can be removed from the
context, using the syntax :module -M.
The import syntax is cumulative (as in a
Haskell module), so this is the only way to subtract from
the scope.

The full syntax of the :module command
is:

:module [+|-] [*]mod1 ... [*]modn

Using the + form of the
module commands adds modules to the current
scope, and - removes them. Without either
+ or -, the current scope
is replaced by the set of modules specified. Note that if you
use this form and leave out Prelude, an
implicit Prelude import will be added
automatically.

After a :load command, an automatic
import is added to the scope for the most recently loaded
"target" module, in a *-form if possible.
For example, if you say :load foo.hs bar.hs
and bar.hs contains module
Bar, then the scope will be set to
*Bar if Bar is
interpreted, or if Bar is compiled it will be
set to Prelude Bar (GHCi automatically adds
Prelude if it isn't present and there aren't
any *-form modules). These
automatically-added imports can be seen with
:show imports:

and the automatically-added import is replaced the next time you
use :load, :add, or
:reload. It can also be removed by
:module as with normal imports.

With multiple modules in scope, especially multiple
*-form modules, it is likely that name
clashes will occur. Haskell specifies that name clashes are
only reported when an ambiguous identifier is used, and GHCi
behaves in the same way for expressions typed at the
prompt.

Hint: GHCi will tab-complete names that are in scope; for
example, if you run GHCi and type J<tab>
then GHCi will expand it to “Just ”.

2.4.5.1. :module and
:load

It might seem that :module and
:load do similar things: you can use both
to bring a module into scope. However, there is a clear
difference. GHCi is concerned with two sets of modules:

The set of modules that are currently
loaded. This set is modified by
:load, :add and
:reload, and can be shown with
:show modules.

The set of modules that are currently in
scope at the prompt. This set is modified by
import, :module, and
it is also modified automatically after
:load, :add, and
:reload, as described above.

You cannot add a module to the scope if it is not
loaded. This is why trying to
use :module to load a new module results
in the message “module M is not
loaded”.

2.4.5.2. Qualified names

To make life slightly easier, the GHCi prompt also
behaves as if there is an implicit import
qualified declaration for every module in every
package, and every module currently loaded into GHCi. This
behaviour can be disabled with the flag -fno-implicit-import-qualified.

2.4.5.3. The :main and :run commands

When a program is compiled and executed, it can use the
getArgs function to access the
command-line arguments.
However, we cannot simply pass the arguments to the
main function while we are testing in ghci,
as the main function doesn't take its
directly.

Instead, we can use the :main command.
This runs whatever main is in scope, with
any arguments being treated the same as command-line arguments,
e.g.:

2.4.6. The it variable

Whenever an expression (or a non-binding statement, to be
precise) is typed at the prompt, GHCi implicitly binds its value
to the variable it. For example:

Prelude> 1+2
3
Prelude> it * 2
6

What actually happens is that GHCi typechecks the
expression, and if it doesn't have an IO type,
then it transforms it as follows: an expression
e turns into

let it = e;
print it

which is then run as an IO-action.

Hence, the original expression must have a type which is an
instance of the Show class, or GHCi will
complain:

Prelude> id
<interactive>:1:0:
No instance for (Show (a -> a))
arising from use of `print' at <interactive>:1:0-1
Possible fix: add an instance declaration for (Show (a -> a))
In the expression: print it
In a 'do' expression: print it

The error message contains some clues as to the
transformation happening internally.

If the expression was instead of type IO a for
some a, then it will be
bound to the result of the IO computation,
which is of type a. eg.:

Note that it is shadowed by the new
value each time you evaluate a new expression, and the old value
of it is lost.

2.4.7. Type defaulting in GHCi

Consider this GHCi session:

ghci> reverse []

What should GHCi do? Strictly speaking, the program is ambiguous. show (reverse [])
(which is what GHCi computes here) has type Show a => String and how that displays depends
on the type a. For example:

ghci> reverse ([] :: String)
""
ghci> reverse ([] :: [Int])
[]

However, it is tiresome for the user to have to specify the type, so GHCi extends Haskell's type-defaulting
rules (Section 4.3.4 of the Haskell 2010 Report) as follows. The
standard rules take each group of constraints (C1 a, C2 a, ..., Cn
a) for each type variable a, and defaults the
type variable if

The type variable a appears in no
other constraints

All the classes Ci are standard.

At least one of the classes Ci is
numeric.

At the GHCi prompt, or with GHC if the
-XExtendedDefaultRules flag is given,
the following additional differences apply:

Rule 2 above is relaxed thus:
All of the classes
Ci are single-parameter type classes.

Rule 3 above is relaxed this:
At least one of the classes Ci is
numeric, or is Show,
Eq, or
Ord.

The unit type () is added to the
start of the standard list of types which are tried when
doing type defaulting.

prints () rather than 0 as the
type is defaulted to () rather than
Integer.

The motivation for the change is that it means IO a
actions default to IO (), which in turn means that
ghci won't try to print a result when running them. This is
particularly important for printf, which has an
instance that returns IO a.
However, it is only able to return
undefined
(the reason for the instance having this type is so that printf
doesn't require extensions to the class system), so if the type defaults to
Integer then ghci gives an error when running a
printf.